WASHINGTON (Reuters) - President Barack Obama and Republicans clashed frequently on Thursday at a summit on his stalled healthcare overhaul, battling over the size and cost of the proposal and moving no closer to a compromise agreement.

Obama told about 40 congressional leaders his comprehensive overhaul was "absolutely critical" to a sustained economic recovery, but Republicans said he should scrap the current plans and start over with a smaller approach.

"There are some fundamental differences between us that we cannot paper over," Jon Kyl, the No. 2 Senate Republican, told Obama, adding his plan gave too much power to Washington and took it away from patients and doctors.

"We do not agree about the fundamental question of who should be in charge," Kyl said.

Obama hoped the day-long summit at Blair House, the presidential guest house across the street from the White House, would revive momentum in Congress for his faltering overhaul.

"I think we're establishing that there are actually some areas of real agreement," Obama told reporters during a lunch break. "And we're starting to focus on what the real disagreements are."

Obama urged lawmakers to go beyond political theater and partisan finger-pointing, but the polite tone was interrupted several times by tense exchanges with Republicans, including his 2008 presidential foe John McCain.

When McCain questioned whether Obama had delivered on the political change he promised, Obama curtly reminded him: "We're not campaigning anymore. The election is over."

"I'm reminded of that every day," McCain said with a laugh.

The summit appeared to have little impact on the health insurance industry in trading on Wall Street. Shares of the Morgan Stanley Healthcare Payor Index and S&P Managed Healthcare Index slightly down but outperforming the overall market.

"We still believe that in the short term whatever reform occurs will be incremental, not comprehensive," said Les Funtleyder, an analyst at the institutional trading firm of Miller Tabak & Co.

'START OVER'

Republican Senator Lamar Alexander and Obama clashed sharply on whether Democratic plans would raise the cost of insurance premiums, with each interrupting the other to make their points.

Republicans said Obama's healthcare overhaul was too costly and would mean more taxes, more regulations and higher premiums for consumers. They stacked the 2,700-page bill on their table to show its size and said their opposition represented the will of a majority of Americans.

"We have to start by taking the current bill and putting it on the shelf and starting from a clean sheet of paper," Alexander said. "This is a car that can't be recalled and fixed."

Obama and his fellow Democrats made it clear they have no intention of starting over, but Obama hopes to influence wavering Democratic lawmakers and rally support among voters who have lost enthusiasm for the effort to reshape the $2.5 trillion U.S. healthcare industry.

The bills passed by the Democratic-controlled House and Senate late last year were designed to rein in costs, regulate insurers and expand coverage to tens of millions of Americans.

But efforts to merge them and send a final version to Obama collapsed in January after Democrats lost their crucial 60th Senate vote in a special election in Massachusetts amid broad public dissatisfaction with the healthcare drive.

Obama offered his own version of the healthcare plan on Monday in an effort to break the legislative gridlock, but Republicans immediately rejected it.

The White House has signaled it would consider backing an effort to ram the bill through Congress using a procedure called reconciliation that would bypass the need for Republican support. Republicans denounced the idea.

You can say that this process has been used before, and that would be right. But it's never been used for anything like this," Alexander said, quoting Democratic Senator Robert Byrd's description of the process as ramming the bill through like "a freight train."

Senate Democratic leader Harry Reid defended the potential use of reconciliation and noted Republicans had used it before "for major things" like tax cuts and reform of Medicare, the federal health program for the elderly.

The White House also has a scaled-back alternative plan it could push if a more comprehensive approach fails. It would extend coverage to about 15 million Americans rather than the 31 million envisioned by the larger plan.

Asked as he entered the summit if he had a Plan B, Obama replied: "I've always got plans."

Republicans focused on promoting their own scaled-back approach to boost competition across state lines, create high-risk insurance pools and curtail medical malpractice lawsuits.